1. Field of the Invention
The present invention generally relates to a data reproducing apparatus such as a magnetic disk apparatus and an optical disk apparatus (including a magneto-optic disk apparatus), and specifically relates to the data reproducing apparatus that restores recorded data according to a predetermined algorithm from a reproduced signal, from which a crosstalk component from at least one adjoining track is removed, the reproduced signal being obtained by a reproducing head scanning recording tracks of a recording medium, such as an optical disk.
2. Description of the Related Art
In order to record data on a recording medium, such as an optical disk (including a magneto-optic disk), in high density, dimensions of recording bits and recording tracks have been miniaturized. The minute bits and the densely provided tracks cause a problem of wave shape interference to a reproduced signal. As a recording/reproducing technique for realizing highly precise data reproduction using this wave interference, a partial response—maximum likelihood (PRML) technique has been developed.
Another problem in a dense track situation such as above includes a cross-write and a crosstalk of a track with adjacent tracks.
The cross-write of, for example, an optical disk apparatus is caused by an optical beam being applied at a power higher than optimum when writing data. The optical beam of the power higher than the optimum protrudes into adjoining tracks beyond a target recording track, resulting in the cross-write. The cross-write deforms information recorded on an adjoining track, and information quality obtained from the recording track at the time of reproduction will deteriorate. The cross-write is avoidable by performing a strobe lighting and an accurate power adjustment of LD (Laser Diode) used as the light source of the optical beam such that heat distribution on an optical disk is controlled with a sufficient precision.
The crosstalk originates from the optical beam spot protruding into adjoining tracks beyond the target recording track at the time of data reproduction. By the protruding, signals from the adjoining tracks mix into a desired signal from the target recording track, which is the crosstalk and will appear as a jitter.
Conventional technology that solves the crosstalk problem has been disclosed, for example, by JP, 5-205280 and JP, 7-254156. There, the crosstalk is canceled by scanning the target recording track and two adjoining tracks thereof simultaneously by using three optical beams, and by deducting unwanted signals from the two adjoining tracks with an appropriate level adjustment from the signal obtained from the target track, which includes the unwanted signals.
However, since the three optical beams are needed with such conventional technology in order to retrieve signals simultaneously from the target recording track and the two adjoining tracks, dimensions of a reading head are enlarged, weight is heavier and cost is higher. Further, it is difficult to align phases between the retrieved signals, differences in the phases being caused by difference in travel distance of the three optical beams.
The JP, 5-205280 has also disclosed technology that cancels the crosstalk using a single optical beam. With this conventional technology, signal acquired from each adjoining track by the single optical beam is sampled by an analog to digital converter, and sampled values are stored beforehand in a memory unit. When a signal from the target recording track is retrieved by the optical beam, cancellation of a crosstalk is made using the sampled values that correspond to the adjoining track.
With such conventional technology, since only one optical beam is used, the optical head can be designed small. Further, there is no need to adjust the phase difference between signals supplied from a plurality of the optical beams.
However, it is difficult to form recording marks of a target recording track and its adjoining tracks in a circumference direction on an optical disk such that there is no phase difference altogether. Further, the target track is scanned by a central part of the optical beam spot, while the adjoining tracks are scanned by peripheral portions of the optical beam spot. For this reason, a phase of a desired signal from the target track is usually different from phases of unwanted signals from the adjoining tracks, which result in the crosstalk.
As described above, the conventional technology has attempted to deduct unwanted signals from retrieved signal, assuming that the phases of the desired signal and the unwanted signals are the same. Because the phases are different in fact, the unwanted signals from the adjoining tracks have not been completely removed from the signal obtained when scanning the target recording track.